The invention relates to improvements in gravure presses and more particularly to improvements in the doctor blade facilities utilized in conjunction with such presses.
In gravure printing, a gravure cylinder is prepared, in which small but defined recesses are provided in the pattern of the image to be printed so that the gravure cylinder has image areas and non-image areas. The gravure cylinder is arranged to cooperate with an impression roll, which forms a pressure nip with the gravure cylinder. A web or sheet of material to be printed passes through the nip, at which time ink, retained in the recesses forming the image areas of the gravure cylinder, is transferred to the surface of the sheet or web.
In accordance with well known practice, printing ink is initially applied to the gravure cylinder, at a point in advance of the transfer nip, over the entire surface of the cylinder. Typically, this may be accomplished by causing the lower portion of the cylinder to operate within an ink pan, with the lower extremities of the gravure cylinder dipping below the surface of the ink in the pan. As the surface of the cylinder emerges from the ink pan, it is fully coated with ink. Thereafter, as the ink-covered surface travels upward, toward the transfer nip, it is wiped by a doctor blade, so that the non-image or-land areas of the cylinder are wiped clean of ink. After passing the doctor blade, the cylinder surface contains ink only in the recesses defining the image area, enabling the ink to be applied to the substrate in accurately predetermined patterns of dots and lines to form the printed image.
As will be readily apparent, quality gravure printing requires that the doctoring operation be performed in a highly effective manner, so that the outer surface (i.e., the non-image areas) of the cylinder is wiped thoroughly clean of ink. Typically, for this purpose, the press is provided with a doctor blade, formed of thin, flat spring steel material, which is mounted to press against the surface of the gravure cylinder a short distance in advance of the transfer nip.
Studies have indicated that for optimal doctoring effectiveness, the doctor blade should be mounted at an angle of about 80.degree. to a tangent line at the point of contact of the doctor blade with the gravure cylinder, the blade making an acute angle with the tangent on the side thereof extending toward the ink supply. Notwithstanding such knowledge, doctor blades are rarely oriented at this optimum angle, because of complications that result from such an orientation. Thus, rarely is there a working gravure press in which the gravure cylinder rotates with perfect trueness. More typically, either the gravure cylinder is slightly eccentric, or the bearings on which the cylinder is mounted introduce eccentricities. In either case, when operating in connection with a doctor blade oriented at the optimum angle of about 80.degree., it is very difficult for the tip of the doctor blade to properly follow the surface of an even slightly eccentric gravure cylinder. This can result in low quality work and/or excessive wear and damage to the gravure cylinder. Gravure cylinders are, of course, very expensive, so that optimum orientation of the doctor blade, although known to be desirable from a theoretical standpoint, has not been put into practice in commercial gravure printing, except perhaps in most expensive, high precision presses, which are relatively few in number.
Instead, conventional gravure presses orient the doctor blade so as to contact the surface of the gravure cylinder at a relatively low angle, for example in the range of 40-60 to the tangent line at the point of contact. While avoiding the problems experienced when the doctor blade is mounted at the optimum (80.degree.) angle, the conventional (40-60) angle gives rise to problems of a different kind. In particular, when a gravure press is operating at high speed, carrying a layer of ink to a doctor blade disposed at a relatively low angle to the cylinder surface, there is a tendency for the doctor blade to hydroplane on the layer of ink. In order to overcome this tendency, significant pressure must be applied to the doctor blade, to cause it to be pressed tightly against the surface of the gravure cylinder. When the cylinder is operating at full speed, during normal printing operations, the heavy pressure of the blade against the cylinder surface is effectively counteracted by the hydraulic forces of the moving ink layer which is being doctored off of the surface of the cylinder. However, when the press is operating at lower speeds, as it frequently is during shading time, and adjusting, etc., the heavy pressure of the doctor blade against the cylinder surface is not counteracted hydraulically, resulting in excessive wearing of the cylinder.
An earlier proposal for dealing with this problem was to provide a doctor blade formed so that its outer end portion is bent at an angle to the main portion of the blade. The bent outer end portion of the blade is thus angled to contact the surface of the gravure cylinder at a relatively optimum angle, while at the same time the main body of the blade lies at a considerably lower angle to the roller in order to provide a desirable degree of flex in the blade. A blade of this configuration is shown and described in British patent specification No. 1,241,554.
In theory, forming the blade with a bent tip provides an ideal arrangement in that the working edge of the blade is disposed at an optimum angle to the gravure roller, while at the same time there is adequate resilience in the mounting of the blade, resulting from the shallower angle of the main body of the blade, to allow the blade to easily follow eccentricities in the gravure cylinder without excessive wear and/or damage.
While the theory of the bent blade configuration of
British patent specification No. 1,241,554 is appealing, it has not, heretofore, proven useful in practice because of incompatibilities of the materials utilized. For example, the standard and preferred hardness of the blade material utilized for the manufacture of doctor blades for this purpose is in the range of 48-52, Rockwell C. Such a material is not capable of being bent at an angle necessary to achieve the objectives of the bent blade concept. Typically, blade material of the prescribed hardness cannot effectively be bent more than about 10 degrees, which is inadequate for the purpose. On the other hand, materials that are of a sufficiently lower hardness to enable the blade tip to be bent at an effective angle to the main body of the blade, for example, at an angle of about 45-60 degrees, are too soft to do an effective job of doctoring the gravure cylinder over an adequate blade working life. Accordingly, notwithstanding that the theoretical advantages of the bent doctor blade have been known, the concept has not achieved any significant level of commercial utilization.
In accordance with the present invention, a novel and improved doctor blade arrangement is provided which effectively enables the desired angular blade configuration to be utilized, while at the same time enabling blade materials of optimum hardness to be employed. This is achieved by constructing the blade, normally a single element, as a three part assembly. This assembly is comprised of an outer or working blade section, which is relatively short and is disposed at an optimum angle to the surface of the gravure roll. A second blade section, which may be referred to as the support section,
is arranged at a desired angle, for example 60 degrees, to the working section. The two blade sections are rigidly joined by a coupling section, advantageously formed of a relatively soft, lightweight material, such as aluminum. The individual blade sections can be, and desirably are of a flat form and are rigidly joined in the desired angular relationship by the coupling section.
In accordance with one of the more specific aspects of the invention, the individual blade sections are secured in the coupling section by a resilient gripping arrangement, which not only simplifies and facilitates the assembly procedures, but also, in operation, provides a desirable degree of vibrational damping.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment and to the accompanying drawings.